Atomic absorption spectrophotometer

ABSTRACT

The invention relates to optical instrment engineering. It is suggested that a liner solid state detector (photodiode array detector, charge coupled device, charge transfer device) that is positioned along the image of entrance slit of the monochrometer be used in an atomic absorption spectrophotometer. This will improve the analytical performance of an atomic absorption spectrophotometer--lower the detection limits, extend the range of concentrations to be measured, increase the reproducibility of measurements by taking into account the temporal and spatial non-uniformities of the analyte distribution over the atomizer volume and will simultaneously provide atomizer walls and of the platform temperatures.

FIELD OF THE INVENTION

The invention relates to optical instruments engineering and may be used when developing atomic absorption (AA) spectrophotometers with electrothermal atomizers.

BACKGROUND ART

At present, Atomic Absorption Spectrometry with electrothermal atomization is one of the most developed methods of determination of trace quantities of more than 60 elements in a variety of substances and materials. High sensitivity, selectivity and precision are characteristic of the method. An inherent feature of this method of analysis is non-stationarity of the analytical signal. All modern spectrophotometers record it with high temporal resolution; however, the signal recorded by a photomultiplier tube (PMT) at the instrument output is essentially an integrated characteristic of analytical volume as a whole and lacks spatial resolution. In the case of a uniform distribution of analyte atoms in the atomizer volume this causes no problems. However, as the imaging of analytical volume has shown, the atoms and molecules of the element to be determined may be distributed within the electrothermal atomizer extremely non-uniformly (1)A. Kh. Gilmutdinov, Yu. A. Zakharov, V. P. Ivanov and A. V. Voloshin. Shadow Spectral Filming: A method of investigating Electrothermal Atomization. Part 1. Dynamics of Formation and Structure of the Absorption Layer of Thallium, Indium, Gallium and Alluminium Atoms.//Journ. Anal. Atom. Spectrometry, 1991, v. 6, p.505-519, (2) A. Kh. Gilmutdinov, Yu. A. Zakharov, V. P. Ivanov, A. V. Voloshin and K. Dittrich. Shadow Spectral Filming: A method of investigating Electrothermal Atomization. Part 2 Dynamics of Formation and Structure of the Absorption Layer of Tallium, Indium, Gallium and Alluminium Molecules.//Journ. Anal. Atom. Spectrometry, 1992, v. 6, p. 675-683.!. On the other hand, theoretical analysis (3) A. Kh. Gilmutdinov, T. M. Abdullina, S. F. Gorbachev and V. L. Makarov. Formation of Analytical Signal in Electrothermal Atomic Absorption Spectrometry. Influence of temperature and concentration gradients.//Zhurn. Analit. Khimii, 1991, v. 46, p. 1480-1492.! shows that the analytical signal recorded at the spectrophotometer output depends not only on the number of absorbing atoms, but also on their distribution over the atomizer cross-section. This leads to two limitations in the existing detection systems. First, the relation between the absorbance recorded at the spectrophotometer output and the number of absorbing atoms proves to be ambiguous. Second, analytical curves appear to be largely curved in the region of high absorbances 3!.

Thus, the use in an AA spectrophotometers of a detection system having no spatial resolution leads to narrowing of the range of concentrations to be measured due to curvature of analytical curves and to the increase of measurement error due to the ignored influence of absorption layers inhomogeneity. Therefore, to obtain a correct analytical signal, it is necessary to have a detection system ensuring not only sufficient temporal but also spatial resolution.

There exist a detection system of a AA spectrophotometer with a linear photodiode array detector (4) Gary P. Moulton, Thomas C. O'Haver, James M. Harnly. Continuum Source Atomic Absorption Spectrometry with a Pulsed Source a Photodiode Array Detector.//Journ. Anal. Atom. Spectrometry, 1989, v. 4, p. 673-674.!. In such a system the photodiode detector is positioned along the spectral instrument dispersion and besides the resonance line also records the radiation of adjacent lines.

However, such an arrangement of the photodiode array detector offers no possibility to use it for spatially resolved recording of the analytical signal. Therefore, such spectrophotometers have no spatial resolution and suffer from the drawbacks similar to the above shortcomings inherent in detection systems based on the use of photomultiplier tubes.

SUMMARY OF THE INVENTION

The object of the invention is to improve the analytical performance of an AA spectrophotometer, namely, lower its detection limits, extend the range of concentrations to be measured, increase the reproducibility of measurements by taking into account the temporal and spatial non-uniformities of the analyte distribution over the atomizer volume and simultaneous measuring of the atomizer walls and of the platform temperatures.

The above object is achieved by positioning a linear solid-state detector (potodiode array detector, charge coupled device, charge transfer device) in an atomic absorption spectrophotometer, consisting of a radiation source, optical illumination system and a monochromator, along the monochromator exit slit in the plane that is optically conjugated to the atomization zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the optical system of a spectrophotometer according to the invention.

FIG. 2 is a cross-sectional view of the atomizer tube of FIG. 1 which illustrates the arrangement of detector pixels relative to the image elements of the atomizer volume.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The concept of the invention is illustrated by the AA spectrophotometer optical diagram (FIG. 1). The AA spectrophotometer comprises a linear spectrum radiation source 1, the illumination system consisting of optical elements 2 and 4, tube atomizer 3, monochromator consisting of entrance slit 5, grating 6 and exit slit 7, objective lens mirror 8 projecting the image of atomization zone onto the linear solid-state detector 9. FIG. 2 shows the arrangement of detector pixels relative to the image elements of the atomizer volume, where 1 is the cross-section of the atomizer tube, 2 is the platform, N₁ . . . N₆ are ordinal numbers of detector array pixels.

The AA spectrophotometer with the solid-state detector operates in the following way. Optical radiation from the primary source 1 is directed by illumination system element 2 to the tube atomizer 3 where it passes through the analyte vapour and is partially absorbed in accordance with the vapour density distribution within the atomizer. The passed radiation is directed by illumination system element 4 to monochromator 5, 6, 7, the parameters of element 4 being selected so that the atomization plane is optically conjugated to monochromator entrance slit 5. Then objective lens 8 directs the radiation to linear photodiode array detector 9. Besides, the detector is spatially oriented in such a way that its array is parallel to the monochromator exit slit knives.

A certain area of the atomization zone corresponds to each detector pixel, i.e. a part of detector pixels measures the absorbance over a small cross-section area of the atomizer zone, whereas the other part measures the intensity of light radiation from the atomizer walls and from the platform. The electric signals received from each detector pixel prior to and during sample atomization are processed to determine the analyte distribution in the atomizer cross-section and absorbances over each elementary cross-section of atomization zone, as well as the temperature of the atomizer wall and of the platform during the whole atomization cycle. Then these elementary absorbances are summed up to obtain the absorbance of the analyte integrated over the furnace cross-section.

Part of the detector pixels records the radiation passed through the atomizer volume filled up with the absorbing atoms. The electric signals generated by these pixels is processed by means of the following equation: ##EQU1##

Here A(t) is the absorbance integrated overt he furnace cross-section at a moment of time t;

I₀ ^(i) is the intensity of translucent emission from the prime source recorded by i-pixel prior to sample atomization;

I_(d) ^(i) is the intensity of dark signal from the detector i-pixel;

I^(i) is the intensity of radiation from the prime source passed through the layer of absorbing atoms and recorded by the detector i-pixel;

N₂ is the number of the pixel to which the upper edge of the atomizer lower wall image corresponds;

N₃ is the number of the pixel to which the lower edge of the platform image corresponds;

N₄ is the number of the pixel to which the upper edge of the platform image corresponds;

N₅ is the number of pixel to which the lower edge of the atomizer upper wall image correpsonds;

The temperature of the graphite wall and of the platform is determined using the brightness method proceeding from the dependence of the heated graphite surface radiation intensity at a given wavelength on its temperature. To determine this dependence, calibration by the readings of a reference pyrometer is effected within the atomizer operating temperature range at each analytical wavelength. The intensity of radiation from a heated graphite surface is calculated from the following formula: ##EQU2## where I_(w) is the total intensity of radiation from the graphite tube walls recorded by linear solid-state detector; I_(p) is the total intensity of radiation from the platform; S(λ) is the function of absolute spectral response of the detector; λ is the wavelength at which the measurement is taken; N₁ is the number of the pixel to which the lower edge of the atomizer lower wall image corresponds;

N₆ is the number of the pixel to which the upper edge of the atomizer upper wall image corresponds.

Then the temperature of the furnace walls and of the platform is determined by means of the formulae:

    I.sub.w f.sub.1 (I.sub.w,λ), T.sub.p f.sub.2 (I.sub.p,λ)

Where T_(w) is the temperature of the furnace wall;

T_(p) is the platform temperature;

f₁ (I_(w),λ) is the functional relation between the intensity of tube walls radiation and the walls temperature for the wavelength λ obtained by calibration against a reference pyrometer;

f₂ (I_(w),λ) is the functional relation between the intensity of the platform radiation and its temperature for the wavelength λ obtained by calibration against a reference pyrometer.

As a result of the use of the present invention in atomic absorption spectrophotometers the following positive effect can be achieved. Measuring atomic absorbances with spatial resolution results in widening of the analytical curve linearity range and provides more accurate analytical results. Measurement of wall and platform temperatures using their radiation in the process of absorbance measurement allows one to obtain more complete information on the process of atomization and eliminates the necessity to use the atomizer temperature control unit. E.g. the additional information on the process of atomization can be utilised to correct the quantity values determined and/or can help to find out the optimal moment of quantity measurement. 

We claim:
 1. An atomic absorption spectrophotometer, comprising a radiation source (1), an illumination system (2,4), an atomizer (3), a monochromator (5-7) and a linear solid-state detector (9) including an array of pixels for detecting optical radiation,wherein the improvement comprises, the linear solid-state detector (9) being arranged along a monochromator exit slit (7), and an image of at least one portion of the atomizer (3) being projected onto the linear solid-state detector so that detector pixels measure the emission of said at least one portion to determine a temperature of the atomizer.
 2. A spectrophotometer according to claim 1, wherein the pixels are arranged adjacent to each other.
 3. A spectrophotometer according to claim 1 wherein the linear solid-state detector (9) is arranged parallel to the edges of the exit slit (7).
 4. A spectrophotometer according to claim 1 wherein an image of an atomization zone is projected onto the linear solid-state detector (9) so that detector pixels measure absorbance over a small cross-section area of the atomization zone.
 5. A spectrophotometer according to claim 1 further comprising an objective lens mirror for projecting an image of an atomization zone onto the linear solid-state detector (9).
 6. A spectrophotometer according to claim 5, wherein the objective lens mirror (8) is positioned between the monochromotor (5-7) and the detector (9).
 7. A spectrophotometer according to claim 1 wherein the atomizer is an electothermal tube atomizer having a platform positioned in the tube.
 8. A spectrophotometer according to claim 7 wherein the emission of portions of at least one of the wall and the platform of the tube atomizer (9) is measured.
 9. A spectrophotometer according to claim 1 wherein the radiation source (1) provides a linear radiation spectrum.
 10. A spectrophotometer according to claim 1, further comprising means for processing the measuring signals of each pixel.
 11. An atomic absorption spectrophotometer comprising:a radiation source; an atomizer; a monochromator having an input and an output; an illumination system positioned so as to direct radiation from said source through said atomizer and then into said monochromator input; a linear solid state detector positioned to receive radiation from said monochromator output; whereby an image of at least one portion of said atomizer is projected onto said solid state detector so that said detector measures the temperature of said atomizer. 